Hydrogen Burning (Proton-proton chain) 1H + 1H → 2H 2H + 1H → 3He 3He + 3He → 4He 1 1 1 1 1 1 2 2 2 2 This process is the longest stage in a star’s life!

Hydrogen Burning The most essential nuclear fusion process in stars The result is the formation of a Helium-4 atom In stars with a low mass, similar to our Sun, hydrogen burning occurs through the proton-proton chain

Nuclear Fusion Step 1 Proton - Proton Fusion

Nuclear Fusion Step 1 Proton - Proton Fusion Start with 2 Hydrogen atoms Hydrogen has only 1 proton

Nuclear Fusion Step 1 Proton - Proton Fusion Start with 2 Hydrogen atoms Hydrogen has only 1 proton As the Hydrogen atoms fuse 1 of the protons becomes a neutron

With a nucleus that has 1 proton and 1 neutron Nuclear Fusion Step 1 Proton - Proton Fusion Start with 2 Hydrogen atoms Hydrogen has only 1 proton As the Hydrogen atoms fuse 1 of the protons becomes a neutron Finish with 1 Hydrogen atom With a nucleus that has 1 proton and 1 neutron

Deuterium - Proton fusion Nuclear Fusion Step 2 Deuterium - Proton fusion Start with Hydrogen -1 and a Hydrogen - 2 Hydrogen -1 is 1 proton nucleus Hydrogen -2 is 1 proton, 1 neutron nucleus

Deuterium - Proton fusion Nuclear Fusion Step 2 Deuterium - Proton fusion Start with Hydrogen -1 and a Hydrogen - 2 Hydrogen -1 is 1 proton nucleus Hydrogen -2 is 1 proton, 1 neutron nucleus Finished with a Helium - 3 Helium -3 is 2 protons and 1 neutron nucleus

Start with Hydrogen -1 and a Hydrogen - 2 Hydrogen -1 is 1 proton nucleus Hydrogen -2 is 1 proton, 1 neutron nucleus Finished with a Helium - 3 Helium -3 is 2 protons and 1 neutron nucleus

Nuclear Fusion Step 3 Helium - 3 fusion

Start with 2 Helium - 3 atoms Helium -3 is 2 protons and 1 neutron nucleus

Start with 2 Helium - 3 atoms Finished with a Helium - 4 atom Helium -3 is 2 protons and 1 neutron nucleus Finished with a Helium - 4 atom Helium -4 is 2 protons and 2 neutrons And 2 free protons

Hydrogen Burning 1H + 1H → 2H 2H + 1H → 3He 3He + 3He → 4He This process is the longest stage in a star’s life!

Hydrogen Burning (Large Stars) In stars much more massive than our Sun, the Hydrogen-1 and deuterium get used up very fast Hydrogen burning occurs through the Carbon-Nitrogen-Oxygen Cycle in these stars

Hydrogen Burning (CNO Cycle)

Fusion of Heavier Elements Once stars burn through all of their “fuel”, heavier elements begin to be fused in the core This is how elements as heavy as iron are formed The fusion of iron takes energy (endothermic) instead of giving off energy (exothermic). Therefore, stars begin to “die” once iron is created.

Carbon Burning 12C + 12C → 16O + 4He + 4He + + 12C + 12C → 20Ne + 4He 8 2 2 + + + + + + + + + + + + + + + + + 12C + 12C → 20Ne + 4He 6 6 10 2 + + + + + + + + + + + + + + + + + + + + + + + + 12C + 12C → 23Na + 2H 6 6 11 1 + + + + + + + + + + + + + + + + + + + + +

Neon Burning 20Ne → 16O + 4He 20Ne + 4He → 24Mg 8 2 20 10 16 8 + 4 2 + 12 20 10 + 24 12 4 2 + +

Oxygen Burning 16O + 16O → 28Si + 4He 16O + 16O → 31P + 1H + + + + + +

Silicon Burning 28Si + 4He → 28S 32S + 4He → 36Ar + + + + + + A chain of alpha processes 28Si + 4He → 28S + + + 32S + 4He → 36Ar + + +

Silicon Burning (cont) 36Si + 4He → 40Ca + + + 40Ca + 4He → 44Ti + + +

Silicon Burning (cont) 44Ti + 4He → 48Cr + + + 48Cr + 4He → 52Fe + + +